So in general the usage for which you are using it is correct from a grammatical standpoint.

That said, common usage is that when "RE:" is the beginning of an email subject line it specifically relates to a reply, and using it otherwise will probably be confusing to email users, many of whom may not be familiar with the term's history or denotation.

It would likely be less confusing if you were to use it in a subject line of an email, but not at the beginning:

I marked yours as accepted, due to the research done and links.
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Devdatta TengsheSep 5 '10 at 12:16

4

Ordinarily, I'd have no objection to Re: meaning "in regard to", but if you're starting a new thread, what's the difference between "Unicorns" and "Re: Unicorns"? Very little, so I'd say leave it off. The only exception would be if you were continuing a discussion started elsewhere, say, at a staff meeting, or in the "Unicorns and Maseratis" thread, then this would indicate reference to what has gone before.
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moiociSep 18 '10 at 7:18

When used in a reply, the field body MAY start with the string "Re: " (from the Latin "res", in the matter of) followed by the contents of the "Subject:" field body of the original message.

So that's the official answer. Note that this specifically links "Re" to a reply.

I do find the explanation for the origin of the phrase suspicious, for the following reasons:

"Subject" already means "in the matter of", more or less, so the "Re" would be redundant.

Internationalized versions of email programs typically translate "Re" to an abbreviation of the translation of "Reply".

Nobody knows that "Re" means "Res", so it's useless to try to make your communication dependent on it.

The original RFC 822 did not contain the specification of "Re", although it did show an example using it. RFC 2822 was published in 2001, but clearly "Re" was in wide use throughout the 1990s, with most people not thinking that it meant "Res".

But if you ignore the parenthetical remark in the standard, what is says is correct and actual practice.

Re: in the subject line of an email means "reply" or "response". Always. So in this context don't use it when you mean "regarding", but when you're replying to an email. Most email applications will add Re: to the subject automatically for you when you click the Reply button.